from decimal import Decimal as D

def S(pos_list):
	data1 = data2 = 0
	for key in range(0, len(pos_list)):
		if key == len(pos_list) - 1:
			data1 += D(str(pos_list[key][0])) * D(str(pos_list[0][1]))
			data2 += D(str(pos_list[key][1])) * D(str(pos_list[0][0]))
		else:
			data1 += D(str(pos_list[key][0])) * D(str(pos_list[key + 1][1]))
			data2 += D(str(pos_list[key][1])) * D(str(pos_list[key + 1][0]))
	S = abs(data1 - data2) / 2
	if S == 0:
		return "这并不是一个图形！"
	else:
		return S